Notes

git_merge

2 min read

Git Merge

Purpose

Merge integrates the history of one branch into another, combining divergent work. It is the standard way to bring a completed feature branch back into main.

Architecture

Git finds the merge base — the best common ancestor commit of the two branch tips — then applies the changes from both branches relative to that base.

Three-way merge (branches have diverged):

A - B - C - F    main
   \       /
    D - E        feature

F is a merge commit with two parents (C and E). It preserves the full branching history.

Fast-forward merge (no divergence — one branch is a direct ancestor of the other):

Before:          After:
A - B     main   A - B - C   main
     \                        feature (deleted)
      C   feature

No merge commit is created; main simply advances to C.

Implementation Notes

Merge a branch into the current branch:

git merge <branch>

Force a merge commit even if fast-forward is possible:

git merge --no-ff <branch>

Abort an in-progress merge (e.g. after conflicts):

git merge --abort

Merge a remote branch:

git merge origin/<branch>

Resolving conflicts: When Git cannot auto-merge, it marks conflict regions in the file:

<<<<<<< HEAD
current branch version
=======
incoming branch version
>>>>>>> feature

Edit the file, then stage and commit:

git add <resolved-file>
git commit

Trade-offs

  • Merge preserves the true branching history — useful for traceability but can make git log noisy.
  • Fast-forward merges produce a clean linear history without extra commits.
  • --no-ff is useful when you want a clear record that a feature branch existed, even if it could fast-forward.
  • For very large teams, squash merges (git merge --squash) collapse feature branch commits into one, keeping main history tidy.

References

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